About the Colloidal Nature and the Osmotic Potential of ASR Products

Although the chemical basics of the ASR are largely revealed and widely accepted, the nature of the ASR expansion mechanism in concrete is still not yet sufficiently well understood. Recent observations showed that ASR products could be considered as colloids. In order to clarify, if and to what extent ASR products exhibit colloidal properties, 10 ASR products of different composition and water content were synthesized and stored for 1.5 years at 40 and 60 ℃. Among others, the ASR products were investigated every 24 weeks by means of NTA, ²⁹Si NMR, XRD and by an osmotic cell test (OCT). The key results show that ASR products contain particles with a diameter of about 50–600 nm, what confirms their colloidal nature. The particle concentration increased with the Si concentration and the degree of polymerization in the ASR products respectively. The particles are unable to pass pores with a size smaller than their own, what represents a mechanism of semi-permeability leading to the Donnan effect and osmosis. By adding Ca, the particles start to link irreversibly to a gel that increasingly loses its osmotic potential. Over time, different crystalline products are formed, depending on composition and temperature.